Synchrotron x-ray radiation sources are of interest for many different fields of science and technology. A synchrotron x-ray radiation source has a wavelength that is tunable. Intense x-ray beams with wavelengths matched to the atomic scale have opened new windows to the physical and biological world. Powerful techniques such as x-ray diffraction and scattering are further enhanced by the tunability of synchrotron radiation that can exploit the subtleties of x-ray spectroscopy.
High flux synchrotrons are typically implemented as centralized facilities that use large magnetic rings to store high-energy electron beams. As an illustrative example, a conventional third generation synchrotron may have a diameter of over 100 meters and utilize a 2–7 GeV beam, which combined with insertion devices such as undulator magnets generate 1 Angstrom wavelength x-ray radiation.
The large physical size, high cost, and complexity of conventional synchrotrons have limited their applications. For example, in many universities, hospitals, and research centers there are limitations on floor space, cost, power, and radiation levels that make a conventional synchrotron impractical as a local source of x-ray radiation. As a result, there are many medical and industrial applications that have been developed using synchrotron radiation that are not widely used because of the unavailability of a practical local source of synchrotron radiation having the necessary x-ray intensity and spectral properties.
Research in compact synchrotron x-ray sources has led to several design proposals for local x-ray sources that use the effect of Compton scattering. Compton scattering is a phenomenon of elastic scattering of photons and electrons. Since both the total energy and the momentum are conserved during the process, scattered photons with much higher energy (light with much shorter wavelength) can be obtained in this way.
One example of a Compton x-ray source is that described in U.S. Pat. No. 6,035,015, “Compton backscattered collimated x-ray source” by Ruth, et al., the contents of which are hereby incorporated by reference. FIG. 1 shows the system disclosed in U.S. Pat. No. 6,035,015. The x-ray source includes a compact electron storage ring 10 into which an electron bunch, injected by an electron injector 11, is introduced by a septum or kicker 13. The compact storage ring 10 includes c-shaped metal tubes 12, 15 facing each other to form gaps 14, 16. An essentially periodic sequence of identical FODO cells 18 surround the tubes 12, 15. As is well known, a FODO cell comprises a focusing quadrupole 21, followed by a dipole 22, followed by a defocusing quadrupole 23, then followed by another dipole 24. The magnets can be either permanent magnets (very compact, but fixed magnetic field) or electromagnetic in nature (field strength varies with external current). The FODO cells keep the electron bunch focused and bend the path so that the bunch travels around the compact storage ring and repetitively travels across the gap 16. As an electron bunch circulates in the ring and travels across a gap 16, it travels through an interaction region 26 where it interacts with a photon or laser pulse which travels along path 27 to generate x-rays 28 by Compton backscattering. The metal tubes may be evacuated or placed in a vacuum chamber.
In the prior art Compton x-ray source of U.S. Pat. No. 6,035,015 a pulsed laser 36 is injected into a Fabry-Perot optical resonator 32. The resonator may comprise highly reflecting mirrors 33 and 34 spaced to yield a resonator period with a pulsed laser 36 injecting photon pulses into the resonator. At steady state, the power level of the accumulated laser or photon pulse in the resonator can be maintained because any internal loss is compensated by the sequence of synchronized input laser pulses from laser 36. The laser pulse repetition rate is chosen to match the time it takes for the electron beam to circulate once around the ring and the time for the photon pulse to make one round trip in the optical resonator. The electron bunch and laser or photon pulses are synchronized so that the light beam pulses repeatedly collide with the electron beam at the interaction region 26.
Special bending and focusing magnets 41, 42, and 43, 44, are provided to steer the electron bunch for interaction with the photon pulse, and to transversely focus the electron beam inside the vacuum chamber in order to overlap the electron bunch with the focused waist of the laser beam pulse. The optical resonator is slightly tilted in order not to block the x-rays 28 in the forward direction, FIG. 1. The FODO cells 18 and the focusing and bending magnets 41, 42 and 43, 44 are slotted to permit bending and passage of the laser pulses and x-ray beam into and out of the interaction region 26. The electron beam energy and circulation frequency is maintained by a radio frequency (RF) accelerating cavity 46 as in a normal storage ring. In addition, the RF field serves as a focusing force in the longitudinal direction to confine the electron beam with a bunch length comparable to the laser pulse length.
In the prior art Compton x-ray source of U.S. Pat. No. 6,035,015 the electron energy is comparatively low, e.g., 8 MeV compared with 3 GeV electron energies in conventional large scale synchrotrons. In a storage ring with moderate energy, it is well-known that the Coulomb repulsion between the electrons constantly pushes the electrons apart in all degrees of freedom and also gives rise to the so-called intra-beam scattering effect in which electrons scatter off of each other. In prior art Compton x-ray sources the laser-electron interaction is used to cool and stabilize the electrons against intra-beam scattering. By inserting a tightly focused laser-electron interaction region 26 in the storage ring, each time the electrons lose energy to the scattered photons and are subsequently re-accelerated in the RF cavity they move closer in phase space (the space that includes information on both the position and the momentum of the electrons), i.e., the electron beam becomes “cooler” since the random thermal motion of the electrons within the beam is less. This laser cooling is more pronounced when the laser pulse inside the optical resonator is made more intense, and is used to counterbalance the natural quantum excitation and the strong intra-beam scattering effect when an intense electron beam is stored. Therefore, the electron beam can be stabilized by the repetitive laser-electron interactions, and the resulting x-ray flux is significantly enhanced.
Conventional Compton x-ray sources have several drawbacks that have heretofore made them impractical in many applications. In particular, prior art compact Compton x-ray sources, such as that described in U.S. Pat. No. 6,035,015, are not sufficiently bright x-ray sources for narrowband applications, such as protein crystallography or phase contrast imaging. Narrowband applications (also known as “monochromatic” applications), are applications or techniques that commonly use a monochromater to filter or select a narrow band of x-ray energies from an incident x-ray beam. As an illustrative example, monochromators typically select less than 0.1% of the relative energy bandwidth. As a result, narrowband applications not only benefit from a source with a high total x-ray source but an x-ray flux that is comparatively bright within a narrow bandwidth.
The x-ray beam of prior art Compton x-ray sources, such as that described in U.S. Pat. No. 6,035,015, has a lower brightness than desired due in part to the large energy spread caused by the electron-laser interaction. The brightness is also less than desired because the optical power level that can be coupled into and stored in the Fabry-Perot cavity between mirrors 33 and 34 for use in Compton backscattering is less than desired, due to a number of limitations on the control, stability, and losses in different elements of the optics system. Additionally, U.S. Pat. No. 6,035,015 has the optical mirror 34 offset from the path of the x-rays to reduce x-ray absorption, resulting in the optical beam being a few degrees off from a true 180 degree backscattering geometry, which significantly reduces Compton backscattering efficiency.
Therefore, what is desired is a compact Compton x-ray source with increased brightness and efficiency that is suitable for narrowband synchrotron radiation applications.